Rubber blends containing silicic acid and comprising additives containing sulphur

ABSTRACT

The present invention relates to a silica-containing rubber mixture produced from at least one rubber, one sulphur-containing alkoxysilane, one crosslinking agent, one filler, and optionally further rubber auxiliary products, wherein this mixture comprises from 0.1 to 15 parts by weight, based on 100 parts by weight of rubber used, of a polysulphide additive of the formula (I)
 
A-S—(S) x —S—B  (I)
     where x is 0, 1, 2, 3 or 4,   and A and B, independently of one another, are an optionally derivatised carboxyalkyl, carboxyaryl or hydroxyaryl group,   and comprises from 0.1 to 15 parts by weight, based on 100 parts by weight of rubber used, of 1,6 -bis(N,N-dibenzylthiocarbamoyldithio)hexane (CAS No.: 151900-44-6).

The present inversion relates to silica-containing rubber mixtures whichcomprise sulphur-containing additives, to processes for the productionof this rubber mixture, to use thereof and to rubber vulcanisatesproduced therefrom and to an additive mixture for rubber.

Many proposals have been devised for successful production of tyres withreduced rolling resistance. DE-A2 255 577 and 4 435 311, EP-A1 0 0670347, and also U.S. Pat. No. A4,709,065 have described particularpolysulphidic silanes as reinforcing additives for silica-containingrubber vulcanisates. However, the use of the polysulphidic silanesdescribed in those documents as reinforcing additives forsilica-containing rubber vulcanisates have the disadvantage that inorder to achieve acceptable processability it is necessary to userelatively large quantities of the expensive polysulphidic silanes, andthat hardness is unsatisfactory.

Other additional substances have been proposed for improving theprocessability of silica-containing rubber mixtures, for example fattyacid esters, fatty acid salts or mineral oils. The additional substancesmentioned have the disadvantage that they increase flowability, but atthe same time reduce the moduli at relatively high elongation (e.g. from100% to 300%) or else reduce the hardness of the vulcanisates, and someof the reinforcing effect of the filler is therefore lost. Insufficienthardness or stiffness of the vulcanisate results in unsatisfactoryrunning performance of the tyre, particularly in curves.

An increase in the quantity added of the reinforcing filler increasesthe hardness of the vulcanisate, but the higher viscosity of the mixtureis disadvantageous for processability, and the same applies to areduction in the quantity of the plasticizing oil.

EP 1 134 253 describes polyether additives for silica-containing rubbervulcanisates which do not exhibit the abovementioned disadvantage ofreducing the modulus. However, the person skilled in the art requires ausage quantity of 8% by weight of the product, based on the rubber, inorder to increase the Shore A hardness value by 3 units. The low modulusat 300% elongation is disadvantageous.

EP 0 489 313 describes additives with good mechanical properties andwith improved hysteresis performance. However, the examples reveal onlyslight, or no, increase of Shore A hardness in comparison with the priorart, bis[3-(triethoxysilyl)propyl]tetrasulphide according to GermanOffenlegungsschrift 2 255 577, and therefore no improvement ofinteraction between polymer and filler.

EP 1 000 968 moreover uses bis[3-(triethoxysilyl)propyl] tetrasulphidesin combination with a specific reversion stabilizer in SBR, where the300-modulus values are very low and therefore inadequate.

EP 0 791 622 B1 describes a rubber composition with at least onediene-based elastomer, filler composed of silica and of carbon black,and also with a silica coupling agent, selected from

-   (i) tetrathiodipropanol polysulphide mixture or-   (ii) combination of tetrathiodipropanol polysulphide and    bis(3-trialkoxysilylalkyl) polysulphide. In particular, the quantity    of tetrathiodipropanol polysulphide is markedly greater than the    quantity of bis(3-trialkoxysilylalkyl)polysulphide, and this is not    advantageous economically because the tetrathiodipropanol    polysulphide is relatively expensive. In addition, the said mixture    exhibits very low tensile strength values. It can be concluded that    the said mixture is too soft (as confirmed by the Shore A values    measured), as reflected in relatively poor running performance of    the tyre, and also reduced lifetime.

It is an object of the present invention to provide rubber mixtureswhich comprise a specific combination of additional substances which donot impair the flowability of rubber mixtures and which providevulcanisates produced therefrom with good properties, in particular inrespect of rolling resistance, abrasion and wet skid performance intyres and the hardness or stiffness of the vulcanisate, with thepossibility of resultant improvement in the running performance oftyres.

Surprisingly, it has now been found that, in combination withsulphur-containing alkoxysilanes1,6-bis(N,N-dibenzylthiocarbamoyldithio)hexane (CAS No.: 151900-44-6)and certain sulphur-containing additives do not adversely affect theflowabiiity of rubber mixtures and lead to vulcanisates with gooddynamic performance, relatively short scorch time, goodhardness/stiffness, markedly improved rolling resistance and, inparticular, less abrasion. The full vulcanization time is moreovermarkedly improved.

It is believed that the synergistic effect results from improvedinteraction between polymer and filler. The invention therefore providesrubber mixtures produced from at least one rubber, onesulphur-containing alkoxysilane, one crosslinking agent, one filler, andoptionally further rubber auxiliary products, and also at least onesulphur-containing additive of the formula (I)A-S—(S)_(x)—S—B  formula (I)where x is 0, 1, 2, 3 or 4,

-   A is a moiety

and

-   B is a moiety

where

-   R¹ to R⁴ are identical or different and are hydrogen, C₁-C₆-alkyl,    C₅-C₆-cycloalkyl, C₆-C₁₀-aryl or a group —CH₂—OR⁵, —CH₂—CH₂—OR⁵,    —NHR⁵, —COR⁵, —CH₂COOR⁵where R⁵═hydrogen, C₁-C₆-alkyl,    C₅-C₆-cycloalkyl, C₆-C₁₀-aryl or C₁-C₆-acyl, and-   R⁶ to R⁷ are identical or different and are hydrogen, C₁-C₆-alkyl,    C₅-C₆-cycloalkyl, C₆-C₁₀-aryl or a group —CH₂—OR⁵, —CH₂—CH₂—OR⁵,    —NHR⁵, —COR⁵, —COOR⁵, —CH₂COOR⁵, where R⁵═hydrogen, C₁-C₆-alkyl,    C₅-C₆-cycloalkyl, C₆-C₁₀-aryl or C₁-C₆-acyl and-   y and z independently of one another are 0, 1 or 2, or-   A and B independently of one another are one of the radicals

in which R¹ to R² are identical or different and are hydrogen,C₁-C₆-alkyl, C₅-C₆-cycloalkyl, C₆-C₁₀-aryl or a group —CH₂—OR⁵,—CH₂—CH₂—OR⁵, —NHR⁵, —COR⁵, —CH₂COOR⁵where R⁵═hydrogen, C₁C₆-alkyl,C₅-C₆-cycloalkyl, C₆-C₁₀-aryl or C₁-C₆-acyl, and from 0.1 to 15 parts byweight, based on 100 parts by weight of rubber used, ofl,6-bis(N,N-dibenzylthiocarbamoyldithio)hexane (CAS No.: 151900-44-6).

It is preferable to use, as polysulphide additive, at least one of thecompounds of the formula (II):

in which

-   x is 0, 1,2, 3 or 4, particularly preferably 2.

It is preferable to use, as polysulphide additive, at least one of thecompounds of the formula (IIa),

in which x is 0, 1, 2, 3 or 4, particularly preferably 2.

It is preferable to use, as polysulphide additive, at least one of thecompounds of the formulae (III), (IIIa), (IIIb).

in which x is 0, 1, 2, 3 or 4, particularly preferably 2.

It is preferable to use, as polysulphide additive, at least one compoundof the formula (IV),

particularly preferably L-cystine, CAS No.: 56-89-3.

The addition of the rubber additive1,6-bis(N,N-dibenzylthiocarbamoyldithio)hexane (CAS No.: 151900-44-6) tobe used according to the invention, and also of at least onesulphur-containing additive of the formula (I), and also the additionof, optionally, other additional substances preferably takes place inthe first portion of the mixing process when temperatures of thecomposition are from 120 to 200° C.; however, it can also take placelater at lower temperatures (from 20 to 120° C.), for example withsulphur crosslinking agent and/or with accelerator. The form in whichthe additive of the invention is added here can be directly that of amixture of the components 1,6-bis(N,N-dibenzylthiocarbamoyldithio)hexane(CAS No.: 151900-44-6) and at least one sulphur-containing additive ofthe formula (I), or can be that of the individual components.

Other preferred polysulphide additives are listed in the Examples.

It is preferable that the silica-containing rubber mixture of theinvention comprises at least one SBR rubber and at least one BR rubber.

It preferably comprises at least one SBR rubber and at least one BRrubber, in a ratio by weight SBR:BR=from 60:40 to 90:10.

It can preferably also comprise at least one NR rubber.

It is preferable that it comprises at least one SBR rubber and at leastone BR rubber and at least one NR rubber in a ratio of at least 60 andat most 85 percent by weight of SBR, based on rubber, and at least 10and at most 35 percent by weight of BR, based on rubber, and at least 5and at most 20 percent by weight of NR, based on rubber.

Both natural rubber and synthetic rubbers are suitable for theproduction of the rubber mixtures of the invention and of the rubbervulcanisates of the invention. Preferred synthetic rubbers are describedby way of example in W. Hofmann, Kautschuktechnologie [Rubbertechnology], Genter-Verlag, Stuttgart 1980.

They encompass inter alia

-   BR— polybutadiene-   ABR— butadiene/C₁-C₄-alkyl acrylate copolymer-   CR— polychloroprene-   IR— polyisoprene-   SBR— styrene/butadiene copolymers with styrene contents of from 1 to    60% by weight, preferably from 20 to 50% by weight-   IIR— isobutylene/isoprene copolymers-   NBR— butadiene/acrylonitrile copolymers with acrylonitrile contents    of from 5 to 60% by weight, preferably from 10 to 50% by weight-   HNBR— partially hydrogenated or fully hydrogenated NBR rubber-   EPDM— ethylene/propylene/diene copolymers,    and also mixtures of these rubbers.

It is preferable that the silica-containing rubber mixture according tothe invention comprises from 0.3 to 7 parts by weight of one or morepolysulphide additives of the formula (I) or of any of the followingformulae derived therefrom, as listed in the claims, based on 100 partsby weight of rubber used and from 0.3 to 7 parts by weight of1,6-bis(N,N-dibenzylthiocarbamoyldithio)hexane (CAS No.; 151900-44-6),based on 100 parts by weight of rubber used.

It is preferable that the quantity of sulphur-containing alkoxysilane isgreater than or equal to the quantity of the polysulphide additive ofthe formula (I).

It is preferable to use the sulphur-containing alkoxysilane in a ratioby weight of from 1.5:1 to 20:1, particularly from 5:1 to 15:1, inrelation to the polysulphide additive of the formula (I).

The rubber mixture of the invention preferably comprises from 0.5 to 5parts by weight, based on 100 parts by weight of rubber used, of apolysulphide additive of the formula (I), and preferably comprises from0.5 to 5 parts by weight of1,6-bis(N,N-dibenzylthiocarbamoyldithio)hexane (CAS No.: 151900-44-6),based on 100 parts by weight of rubber used.

The present invention further provides rubber vulcanisates which can beproduced from the rubber mixtures of the invention.

The present invention further provides a process for the production offilled rubber vulcanisates, which is characterized in that

-   i) at least one robber is mixed with-   ii) from 10 to 150% by weight, preferably from 30 to 120% by weight,    based on rubber (i), of filler and-   iii) from 0.1 to 15% by weight, preferably from 0.3 to 7% by weight,    based on rubber (i), of polysulphide additives of the formula (I)    and-   iv) from 0.1 to 15% by weight, preferably from 0.3 to 7% by weight,    of 1,6-bis(N,N-dibenzylthiocarbamoyldithio)hexane (CAS No.:    151900-44-6), based on rubber (i) where temperatures of the    composition are at least 120° C. and shear rates are from 1 to 1000    sec (exp.−1), preferably from 1 to 100 sec (exp.−1) and the mixture    is then vulcanized conventionally after addition of further    vulcanization chemicals.

It is preferable that the addition of the inventive polysulphideadditives of the formula (I), and also the addition of1,6-bis(N,N-dibenzylthiocarbamoyldithio)hexane (CAS No.: 151900-44-6)takes place in the first portion of the mixing process wheretemperatures of the composition are from 100 to 200° C. and shear ratesare as mentioned, but it can also take place later at lower temperatures(from 40 to 100° C.), for example together with sulphur and accelerator.

The form in which the polysulphide additives of the formula (I) and1,6-bis(N,N-dibenzylthiocarbamoyldithio)hexane (CAS No.: 151900-44-6)are added to the mixing process can mutually independently be eitherpure form or else a form absorbed on inert, organic or inorganiccarriers. Preferred carrier materials are silica, natural or syntheticsilicates, aluminium oxide and/or carbon blacks.

It is also possible that the polysulphide additives of the formula (I)are added to the mixing process in the form of a mixture with1,6-bis(N,N-dibenzylthiocarbamoyldithio)hexane (CAS No.: 151900-44-6).

For the purposes of this invention, silica-containing fillers that canbe used for the rubber mixture and rubber vulcanisates of the inventionare the following fillers:

-   -   fine-particle silica, produced for example by precipitation from        solutions of silicates or flame hydrolysis of silicon halides        with specific surface areas of from 5 to 1000 m²/g, preferably        from 20 to 400 m²/g (BET surface area) and with primary particle        sixes of from 10 to 400 nm. The silicas can optionally also take        the form of mixed oxides with other metal oxides, such as Al,        Mg, Ca, Ba, Zn, Zr, Ti oxides.    -   Synthetic silicates, such as aluminium silicate, alkaline earth        metal silicates, such as magnesium silicate or calcium silicate,        with BET surface areas of from 20 to 400 m²/g and primary        particle size of from 10 to 400 nm,    -   natural silicates, such as kaolin and other naturally occurring        silicas,    -   glass fibres and glass-fibre products (mats, strands) or glass        microbeads.

Other fillers that can be used are carbon blacks. The carbon blacks tobe used here are produced by way of example by the lamp-black process,furnace-black process or gas-black process and have BET surface areas offrom 20 to 200 m²/g, examples being SAF, ISAF, IISAF, HAF, FEF, or GPFcarbon blacks.

Preferred quantities used of the polysulphide additives of the formula(I) in the rubber mixtures of the invention are from 0.3 to 7%, based onrubber.

One particularly preferred variant consists in the combination ofsilica, carbon black and polysulphide additives of the formula (I), Theratio of silica to carbon black in this combination can be varied asdesired. From the point of view of tyre technology, preference is givento a silica: carbon black ratio of from 20:1 to 1.5:1.

Sulphur-containing silanes that can be used for the rubber vulcanisatesaccording to the invention are preferably bis(triethoxysilylpropyl)tetrasulphane and the corresponding disulphane and3-triethoxysilyl-1-propanethiol or silanes such as Si 363 from Evonik,Germany or silane NXT or NXT Z from Momentive (previously GE, USA),where the alkoxy moiety is methoxy or ethoxy where quantities used arefrom 2 to 20 parts by weight, preferably from 3 to 11 parts by weight,calculated in each case as 100% strength active ingredient and based on100 parts by weight of rubber. However, it is also possible to use amixture made of the said sulphur-containing silanes. Liquidsulphur-containing silanes can have been absorbed on a carrier toimprove ease of metering and/or ease of dispersion (dry liquid). Activeingredient content is from 30 to 70 parts by weight, preferably from 40to 60 parts by weight, for every 100 parts by weight of dry liquid.

The rubber vulcanisates according to the invention can comprise otherrubber auxiliary products, for example reaction accelerators,antioxidants, heat stabilisers, light stabilizers, antiozonants,processing aids, plasticizers, tackifiers, blowing agents, dyes,pigments, waxes, extenders, organic acids, retardants, metal oxides, andalso activators, such as triethanolamine, polyethylene glycol,hexanetriol, where these are known to the rubber industry.

The quantity used of the rubber auxiliary products is conventional anddepends inter alia on the intended purpose of the vulcanisates.Conventional quantities, based on rubber, are from 0.1 to 30% by weight.

The following are used as crosslinking agents: peroxides, sulphur,magnesium oxide, zinc oxide, and the known vulcanisation acceleratorscan also be added to these, for example mercaptobenzothiazoles,-sulphenamides, thiurams, thiocarbamates, guanidines, xanthogenates andthiophosphates. Preference is given to sulphur.

The quantities used of the crosslinking agents and vulcanizationaccelerators are about 0.1 to 10% by weight, preferably 0.1 to 5% byweight, based on rubber.

As mentioned above, it is advantageous to add antioxidants to the rubbermixture to counteract the effect of heat and oxygen. Suitable phenolicantioxidants are alkylated phenols, styrenated phenol, stericallyhindered phenols such as 2,6-di-tert-butylphenol,2,6-di-tert-butyl-p-cresol (BHT), 2,6-di-tert-butyl-4-ethylphenol,sterically hindered phenols containing ester groups, sterically hinderedphenols containing thioether,2,2′-methylenebis(4-methyl-6-tert-butylphenol) (BPH), and alsosterically hindered thiobisphenols.

If discoloration of the rubber is not of importance, it is also possibleto use aminic antioxidants, e.g. mixtures of diaryl-p-phenylenediamines(DTPD), octylated diphenylamine (ODPA), phenyl-α-naphtyhlamine (PAN),phenyl-β-naphthylamine (PBN), preferably those based onphenylenediamine. Examples of phenylenediamines areN-isopropyl-N′-phenylenediamine,N-1,3-dimethylbutyl-N′-phenyl-p-phenylenediamine (6PPD),N-1,4-dimethylpentyl-N′-phenyl-p-phenylenediamine (7PPD),N,N′-bis(1,4-dimethylpentyl)-p-phenylenediamine (77PD).

Among the other antioxidants are phosphites such astris(nonylphenyl)phosphite, polymerized2,2,4-trimethyl-1,2-dihydroquinoline (TMQ), 2-mercaptobenzimidazole(MBI), methyl-2-mercaptobenzimaidazole (MMBI), zincmethylmercaptobenzimidazole (ZMMBI). The phosphites are generally usedin combination with phenolic antioxidants. TMQ, MBI and MMBI are mainlyused for NBR types which are vulcanized peroxidically.

Ozone resistance can be improved by using antioxidants known to a personskilled in the art, such asN-1,3-dimethylbutyl-N′-phenyl-p-phenylenediamine (6PPD),N-1,4-dimethylpentyl-N′-phenyl-p-phenylenediamine (7PPD),N,N′-bis(1,4-dimethylpentyl)-p-phenylenediamine (77PD), enol ethers orcyclic acetals.

Processing aids are intended to act between the rubber particles and tocounteract frictional forces during the mixing, plastification andshaping process. Processing aids which can be present in the rubbermixture according to the invention are any of the lubricantsconventionally used for the processing of plastics, for examplehydrocarbons, such as oils, paraffins and PE waxes, fatty alcoholshaving from 6 to 20 carbon atoms, ketones, carboxylic acids, such asfatty acids and montanic acids, oxidised PE wax, metal salts ofcarboxylic acids, carboxamides and carboxylic esters, for example withthe following alcohols: ethanol, fatty alcohols, glycerol, ethanediol,pentaerythritol, and long-chain carboxylic acids as acid component.

The rubber mixture can be crosslinked not only with sulphur acceleratorsystems but also with peroxides.

Examples of crosslinking agents that can be used are peroxidiccrosslinking agents such as bis (2,4-dichlorobenzyl) peroxide, dibenzoylperoxide, bis(4-chlorobenzoyl) peroxide, 1,1-bis(tert-butylperoxy)-3,3,5-tremethylcyclohexane, tert-butyl perbenzoate,2,2-bis(tert-butylperoxy)butene, 4,4-di-tert-butyl peroxynonylvalerate,dicumyl peroxide, 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane,tert-butylcumyl peroxide, 1,3-bis-(tert-butylperoxyisopropyl) benzene,di-tert-butyl peroxide and2,5-dimethyl-2,5-di(tert-butylperoxy)hex-3-yne.

It can be advantageous to use, alongside the said peroxidic crosslinkingagents, further additions which can be used to increase crosslinkingyield: a suitable example here being triallyl isocyanurate, triallylcyanurate, trimethylolpropane tri(meth)acrylate, triallyl trimellitate,ethylene glycol dimethacrylate, butanediol dimethacrylate,trimethylolpropane trimethacrylate, Zn diacrylate, Zn dimethacrylate,1,2-polybutadiene or N,N′-m-phenylenedimaleimide.

Another crosslinking agent that can be used is sulphur in elementalsoluble or insoluble form or sulphur donors.

Examples of sulphur donors that can be used are dimorpholyl disulphide(DTDM), 2-morpholino-dithiobenzothiazole (MBSS), caprolactam disulphide,dipentamethylenethiuram tetrasulphide (DPTT), and tetramethylthiuramdisulphide (TMTD).

For the sulphur-vulcanization of the rubber mixture according to theinvention, it is also possible to use further additions which can beused to increase crosslinking yield. In principle, however, it is alsopossible to use sulphur or sulphur donors alone for crosslinking.

Examples of suitable additions which can be used to increasecrosslinking yield are dithiocarbamates, thiurams, thiazoles,sulphenamides, xanthogenates, bi- or polycyclic amines, guanidinederivatives, dithiophosphates, caprolactams and thiourea derivatives.

Examples of equally suitable additions are: diamine zinc diisocyanate,hexamethylenetetramine, 1,3-bis-(citraconimidomethyl)benzene and alsocyclic disulphanes.

Preference is given to the sulphur accelerator system in the rubbermixture according to the invention.

In order to reduce flammability and to reduce smoke generation duringcombustion, the rubber mixture composition according to the inventioncan also comprise flame retardants. An example of a flame retardant usedis antimony trioxide, phosphoric esters, chloroparaffin, aluminiumhydroxide, boron compounds, zinc compounds, molybdenum trioxide,ferrocene, calcium carbonate or magnesium carbonate.

The rubber vulcanisate can also comprise other synthetic polymers,acting by way of example as polymeric processing aids or impactmodifiers. The said synthetic polymers are selected from the groupconsisting of the homo- and copolymers based on ethylene, propylene,butadiene, styrene, vinyl acetate, vinyl chloride, glycidyl acrylate,glycidyl methacrylate, acrylates and methacrylates having alcoholcomponents of branched or unbranched C1-C10-alcohols. Particular mentionmay be made of polyacrylates having identical or different alcoholmoieties from the group of the C4-C8-alcohols, particularly of butanol,hexanol, octanol and 2-ethylhexanol, polymethyl methacrylate, methylmethacrylate-butyl acrylate copolymers, methyl methacrylate-butylmethacrylate copolymers, ethylene-vinyl acetate copolymers, chlorinatedpolyethylene, ethylene-propylene copolymers, ethylene-propylene-dienecopolymers.

The rubber vulcanisate according to the invention can be used forproducing foams. For this, chemical or physical blowing agents areadded. Chemical blowing agents that can be used are any of thesubstances known for this purpose, for example azodicarbonamide,p-toluolsulphonyl hydrazide, 4,4′-oxybis(benzenesulphonyl hydrazide),p-toluenesulphonylsemicarbazide, 5-phenyltetrazole,N,N′-dinitrosopentamethylenetetramine, zinc carbonate or sodiumhydrogencarbonate, and also mixtures comprising these substances. Anexample of a suitable physical blowing agent is carbon dioxide orhalogenated hydrocarbons.

The vulcanization process can take place at temperatures of from 100 to200° C., preferably from 130 to 180° C., optionally under a pressure offrom 10 to 200 bar.

The blending of the rubber with the filler and with the polysulphideadditives of the formula (I) can be carried out in/on conventionalmixing assemblies, for example rolls, internal mixers and mixingextruders.

The rubber vulcanisates according to the invention are suitable forproducing mouldings with improved properties, e.g. for producing cablesheathing, hoses, drive belts, conveyor belts, roll coverings, tyres,shoe soles, sealing rings and damping elements.

An important factor in the processing of rubbers is that the rubbermixture initially prepared with the additives has low flow viscosity(Mooney viscosity ML 1+4/100° C.), so that it is easy to process. Inmany applications, the intention is that the vulcanization process whichfollows (for example at 170° C., t95) for the rubber mixture is toproceed as rapidly as possible with exposure to heat, in order torestrict the cost of time and of energy.

The shaping-dependent scorch time (for example t5) is intended to bewithin a narrow range of times, in particular >300 seconds and <500seconds.

It is preferable that the loss factor tan delta of a vulcanisateproduced from the silica containing rubber mixture according to theinvention under heating conditions 170° C./t95 is <0.13 at 60° C. andthat the Shore A hardness thereof is simultaneously >60 at 23° C., andit is particularly preferable that the loss factor tan delta is <0.12 at60° C. and that the shore A hardness is simultaneously >60 at 23° C. The300 modulus value of the vulcanisate is >12 MPa, preferably >15 MPa, inparticular >20 MPa.

It is preferable that the loss factor tan delta of a vulcanisateproduced from the silica-containing rubber mixture under heatingconditions 170° C./t95 is less than 0.12 at 60° C. and that its scorchtime is simultaneously greater than 300 seconds, hut less than 1000seconds.

It is preferable that the loss factor tan delta of a vulcanisateproduced from the silica-containing rubber mixture under heatingconditions 170° C./t95 is less than 0.12 at 60° C. and that its fullvulcanisation time is simultaneously less than 1000 seconds, preferablyless than 500 seconds.

It is preferable that the scorch time of a vulcanisate produced from thesilica-containing rubber mixture under heating conditions 170° C./t95 isgreater than 300 seconds and that its full vulcanisation time issimultaneously less than 500 seconds.

The invention further provides the use of the silica-containing rubbermixture of the invention for the production of vulcanisates and rubbermouldings of any type, in particular for the production of tyres andtyre components.

The invention further provides an additive mixture comprising one ormore polysulphide additives of the formula (I) and1,6-bis(N,N-dibenzylthiocarbamoyldithio)hexane (CAS No.: 151900-44-6).

It is preferable that the additive mixture comprises from 10 to 90 partsby weight of one or more polysulphide additives of the formula (I),based on 100 parts by weight of additive mixture, and from 10 to 90parts by weight, based on 100 parts by weight of additive mixture, of1,6-bis(N,N-dibenzylthiocarbamoyldithio)hexane (CAS No.: 151900-44-6).

It is preferable that the ratio by weight of one or more polysulphideadditives of the formula (I) to1,6-bis(N,N-dibenzylthiocarbamoyldithio)hexane (CAS No.: 151900-44-6) inthe additive mixture of the invention is from 10:90 to 90:10, inparticular from 25:75 to 75:25.

The automobile industry has been searching for cost-effective ways ofreaching the target of no more than 130 g/km of CO₂ emission, at leastsince the European Union has been concerned with limits for carbondioxide emission from cars. Low-rolling-resistance tyres are ofsubstantial importance here. They reduce fuel consumption by requiringless energy for deformation during freewheeling.

In order that the reduction of rolling resistance is not achieved at thecost of other important properties, the requirements relating to wetgrip and rolling noise are also simultaneously defined. A firstindication of wet skid performance and roiling resistance is givers bythe loss factor tan delta. This should be as high as possible at 0° C.(good wet skid resistance) and as low as possible at from 60 to 70° C.(low rolling resistance). The hardness of a rubber vulcanisate gives afirst indication of its stiffness.

POLYSULPHIDE ADDITIVE EXAMPLES Example 1

-   Apparatus: 500 ml four-necked flask with thermometer, dropping    funnel with pressure equalization, reflux condenser with    gas-discharge attachment (bubble counter) and tubing, stirrer,-   Initial charge: 91.75 g=0.75 mol of methyl 3-mercaptopropionate    (Acros, ≧98%) 250 ml of cyclohexane (analytical grade, from Merck,    dried over molecular sieve)-   Feed: 51.15 g=0.375 mol of disulphur dichloride (≧99%, from Merck)

In the apparatus flushed with nitrogen, dried cyclohexane and methyl3-mercaptopropioniate are introduced. When the methyl3-mercaptopropionate is fully dissolved, the disulphur dichloride isadded dropwise in the course of about 1 hour at a temperature of 5-10°C., during which nitrogen is passed through the mixture. The meteringrate should be set such that a temperature of 10° C. is not exceeded.

After the end of the reaction, stirring is continued, still withnitrogen being passed through, at room temperature overnight.

The reaction solution is subsequently concentrated by evaporation on aRotavapor at 50° C. and the product is dried to constant weight in avacuum drying oven at 60° C.

-   Yield: 108.4 g (95.6%) of a polysulphide mixture of the idealised    formula

Example 2

-   Apparatus: 2000 ml four-necked flask with thermometer, dropping    funnel with pressure equalization, reflux condenser with    gas-discharge attachment (bubble counter) and tubing, stirrer,    gas-inlet tube-   Initial charge: 118.0 g=0.75 mol of mercaptobenzoic acid (Aldrich,    ≧99%) 900 ml of toluene (analytical grade, from Aldrich, dried on a    molecular sieve)-   Feed: 51.15 g=0.375 mol of disulphide dichloride (≧99%, from Merck)

In the apparatus flushed with nitrogen, dried toluene andmercaptobenzoic acid are introduced. The suspension present is thenadmixed dropwise with the disulphur dichloride over the course of about1 hour at a temperature of 0-5° C., with nitrogen being passed through.The metering rate should be set such that a temperature of 5° C. is notexceeded.

After the end of reaction, stirring is continued, with nitrogen beingpassed through, at room temperature overnight.

The reaction solution is subsequently filtered through a D4 frit and thefilter product is rinsed 2× with about 200 ml of dried toluene. Theproduct is dried in a vacuum drying oven at room temperature (about 25°C.).

-   Yield: 144.6 g (104.1%) of a polysuiphide mixture of the idealized    formula

Example 3

Results

The examples below provide further explanation of invention, but thereis no intention that the invention be restricted thereby.

The following rubber formulations, listed in Table 1, were selected forthe tests. Unless otherwise stated, all numeric data are based on “partsper hundred rubber” (phr).

The following rubber mixtures were produced in a 1.5 L internal mixer(70 rpm), start temperature 80° C., mixing time: 5 minutes. Sulphur andaccelerator were finally admixed on a roll (temperature: 50° C.).

TABLE 1 Rubber formulation Reference Rubber formulation 1 Rubberformulation 2 BUNA CB 24 30 30 30 BUNA VSL 5025-1 96 96 96 CORAX N 3396.4 6.4 6.4 VULKASIL S 80 80 80 TUDALEN 1849-1 8 8 8 EDENOR C 18 98-1001 1 1 VULKANOX 4020/LG 1 1 1 VULKANOX HS/LG 1 1 1 ROTSIEGEL ZINC WHITE2.5 2.5 2.5 ANTILUX 654 1.5 1.5 1.5 SI 69 6.4 6.4 6.4 VULKACIT D/C 2 2 2VULKACIT CZ/C 1.5 1.5 1.5 CHANCEL 90/95 GROUND 1.5 1.5 1.5 SULPHURVulcuren 1 1 Example 1 1 Example 2 1 Non-inventive comparisonsComparison 1 Comparison 2 Comparison 3 BUNA CB 24 30 30 30 BUNA VSL5025-1 96 96 96 CORAX N 339 6.4 6.4 6.4 VULKASIL S 80 80 80 TUDALEN1849-1 8 8 8 EDENOR C 18 98-100 1 1 1 VULKANOX 4020/LG 1 1 1 VULKANOXHS/LG 1 1 1 ROTSIEGEL ZINC WHITE 2.5 2.5 2.5 ANTILUX 654 1.5 1.5 1.5 SI69 6.4 6.4 6.4 VULKACIT D/C 2 2 2 VULKACIT CZ/C 1.5 1.5 1.5 CHANCEL90/95 GROUND 1.5 1.5 1.5 SULPHUR Example 1 1 Example 2 1 Vulcuren 1 BUNACB 24 BR Lanxess Deutschland GmbH BUNA VSL 5025-1 SBR LanxessDeutschland GmbH CORAX N 339 Carbon black Degussa-Evonik GmbH VULKASIL SSilica Lanxess Deutschland GmbH TUDALEN 1849-1 Mineral oilHansen&Rosenthal KG EDENORC 18 98-100 Stearic acid Cognis DeutschlandGmbH VULKANOX 4020/LG N-1,3-dimethybutyl-N-phenyl-p- Lanxess DeutschlandGmbH phenylenediamine VULKANOX HS/LG Polymerized 2,2,4-trimethyl-1,2-Lanxess Deutschland GmbH dihydroquinoline ROTSIEGEL ZINC Zinc oxideGrillo Zinkoxid GmbH WHITE ANTILUX 654 Light-stabilizer wax RheinChemieRheinau GmbH Si 69 bis(triethoxysilylpropyl) Evonik Industriestetrasulphide VULKACIT D/C 1,3-Diphenylguanidine Lanxess DeutschlandGmbH VULKACIT CZ/C N-cyclohexyl-2-benzothiazole- Lanxess DeutschlandGmbH sulphenamide CHANCEL 90/95 Sulphur Solvay Deutschland GmbH GROUNDSULPHUR Vulcuren 1,6,bis(N,N-dibenzyl- Lanxess Deutschland GmbHthiocarbamoyldithio)hexane

TABLE 2 Collation of results Rubber Rubber Parameter Unit DIN Referenceformulation 1 formulation 2 Mooney viscosity [MU] 53523 95 125 147 (ML1 + 4) Mooney scorch at sec acc. to 1253 346 461 130° C. (t5) ASTM D5289-95 Full vulcanization at s 53529 1417 246 290 170° C./t95 Shore Ahardness at [Shore A] 53505 66 65 65 23° C. 300 modulus MPa 53504 15 2121 Elongation at break % 53504 349 327 303 Tensile strength MPa 53504 1919 21 Abrasion mm³ 53516 74 64 65 Wet skid performance — 0.463 0.3860.437 (tan d (0° C.)) Rolling resistance — 0.133 0.116 0.111 (tan d (60°C.))

Surprisingly, as shown by the results in Table 2, it was found that thescorch time (130° C./t5) measured in the Inventive Examples (rubberformulation 1 and rubber formulation 2) was significantly shorter whencomparison was made with the reference. Mechanical properties such astensile strength, elongation at break and in particular 300 modulus arevery good. The vulcanisates tested show good wet skid performance andmarkedly improved rolling resistance in relation to the reference (tandelta at 0° C.>0.35 and tan delta at 60° C.<0.13) and likewise veryadvantageous abrasion values (<70 mm³). There was a marked improvementin full vulcanization time.

TABLE 2a Collation of results of comparisons Comparison ComparisonComparison Parameter Unit DIN 1 2 3 Mooney viscosity (ML 1 + 4) [MU]53523 82 92 106 Mooney scorch at 130° C. (t5) sec acc. to 1244 1228 551ASTM D 5289-95 Full vulcanization at 170° C./t95 s 53529 1617 1315 262Shore A hardness at 23° C. [Shore A] 53505 72 73 65 300 modulus MPa53504 17 18 19.5 Elongation at break % 53504 346 308 313 Tensilestrength MPa 53504 20 18 20.5 Abrasion mm³ 53516 95 93 78 Wet skidperformance (tan d (0° C.)) — 0.444 0.452 0.434 Rolling resistance —0.168 0.154 0.112 (tan d (60° C.))

Surprisingly, as shown by the results in Table 2 and 2a, the scorch time(130° C./t5) measured in the Inventive Examples was found to besignificantly shorter than in the Comparative Examples. The vulcanisatesof the invention show better 300 modulus values and excellent rollingresistance. The very advantageous abrasion values (<70 mm³) of theInventive Examples are particularly surprising. Abrasion was improved by20% in the Inventive Examples when comparison is made with ComparativeExample 3. Measured on the basis of the abrasion values from ComparativeExamples 1 and 2, the improvement potential achieved with the InventiveExamples is more than 40%.

Testing of the Rubber Mixture and of the Vulcanisates:

Mooney Viscosity Measurement:

Viscosity can be determined directly from the resisting force exerted bythe rubbers (and rubber mixtures) while they are processed. In theMooney shearing-disc viscometer a grooved disc is surrounded above andbelow by sample substance and is rotated at about two revolutions perminute in a beatable chamber. The force required for this purpose ismeasured in the form of torque and corresponds to the respectiveviscosity. The specimen is generally preheated to 100° C. for 1 minute;the measurement takes a further 4 minutes, while the temperature is heldconstant.

The viscosity is given together with the respective test conditions, anexample being ML (1+4) 100° C. (Mooney viscosity, large rotor, preheattime and test time in minutes, test temperature).

The viscosities of the rubber mixtures specified in Table 1 are measuredby means of a Mooney shearing-disc viscometer.

Scorch Performance (t5 Scorch Time):

The same test can also be used as described above to measure the“scorch” performance of a mixture. The temperature selected in thisPatent is 130° C. The rotor runs until, after the torque value haspassed through a minimum, it has risen to 5 Mooney units relative to theminimum value (t5). The greater the value (the unit here being seconds),the slower the scorch (high scorch values here).

Rheometer (Vulcameter) 170° C./t95 Full Vulcanization Time:

The progress of vulcanization in a MDR (moving die rheometer) andanalytical data therefor are measured in accordance with ASTM D5289-95in a MDR 2000 Monsanto rheometer. Table 2 collates the results of thistest.

The time at which 95% of the rubber has crosslinked is measured as thefull vulcanization time. The temperature selected was 170° C.

Determination of Hardness

In order to determine the hardness of the rubber mixture according tothe invention, milled sheets of thickness 6 mm made of the rubbermixture were produced according to formulations from Table 1. Testspecimens of diameter 35 mm were cut from the milled sheets, and theShore A hardness values were determined for these by means of a digitalShore hardness tester (Zwick GmbH & Co. KG, Ulm).

Tensile Test:

The tensile test serves directly to determine the loading limits of anelastomer. The longitudinal elongation at break is divided by theinitial length to give the elongation at break. The force required toreach certain stages of elongation, mostly 50, 100, 200 and 300%, isalso determined and expressed as modulus (tensile strength at the givenelongation of 300%, or 300 modulus).

Table 2 lists the test results.

Dyn, Damping:

Dynamic test methods are used to characterise the deformationperformance of elastomers under loadings which change periodically. Anexternal stress changes the conformation of the polymer chain.

This measurement determines the loss factor tan delta indirectly by wayof the ratio between loss modulus G″ and storage modulus G′.

What is claimed is:
 1. A silica-containing rubber mixture comprising: arubber, a sulphur-containing alkoxysilane, a crosslinking agent, afiller, 0.1 to 15 parts by weight, based on 100 parts by weight of therubber, of a polysulphide additive of the formula (I)A—S—(S)_(x)—S—B  (I) where x is 0, 1, 2, 3 or 4, A is a moiety

and B is a moiety

where R¹ to R⁴ are identical or different and are hydrogen, C₁-C₆-alkyl,C₅-C₆-cycloalkyl, C₆-C₁₀-aryl or a group —CH₂—OR⁵, —CH₂—CH₂—OR⁵, —NHR⁵,—COR⁵, —COOR⁵, —CH₂COOR⁵, where R⁵=hydrogen, C₁-C₆-alkyl, C₅-C₆-cycloalkyl, C₆-C₁₀-aryl or C₁-C₆-acyl, R⁶ R⁷ are identical ordifferent and are hydrogen, C₁-C₆-alkyl, C₅-C₆-cycloalkyl, C₆-C₁₀-arylor a group —CH₂—OR⁵, —CH₂—CH₂—OR⁵, —NHR⁵, —COR⁵, —COOR⁵, —CH₂COOR⁵,where R⁵=hydrogen, C₁-C₆-alkyl, C₅-C₆-cycloalkyl, C₆-C₁₀-aryl orC₁-C₆-acyl, and y and z independently of one another are 0, 1 or 2, or Aand B independently of one another are one of the radicals

in which R¹ and R² are identical or different and are hydrogen,C₁-C₆-alkyl, C₅-C₆-cycloalkyl, C₆-C₁₀-aryl or a group —CH₂—OR⁵,—CH₂—CH₂—OR⁵, —NHR⁵, —COR⁵, —COOR⁵, —CH₂COOR⁵, where R⁵=hydrogen,C₁-C₆-alkyl, C₅-C₆-cycloalkyl, C₆-C₁₀-aryl or C₁-C₆-acyl, and 0.1 to 15parts by weight, based on 100 parts by weight of the rubber, of1,6-bis(N,N-dibenzylthiocarbamoyldithio)hexane (CAS No.: 151900-44-6).2. The silica-containing rubber mixture according to claim 1, whereinthe polysulphide additive is a compound of the formula (II),

where x is 0, 1, 2, 3 or
 4. 3. The silica-containing rubber mixtureaccording to claim 1, wherein the polysulphide additive is a compound ofthe formula (IIa),

where x is 0, 1, 2, 3 or
 4. 4. The silica-containing rubber mixtureaccording to claim 1, wherein the polysulphide additive is a compound ofthe formula (III), (IIIa), (IIIb),

where x is 0, 1, 2, 3 or 4, particularly preferably
 2. 5. Thesilica-containing rubber mixture according to claim 1, wherein thepolysulphide additive is a compound of the formula (IV)


6. The silica-containing rubber mixture according to claim 1, wherein aquantity of sulphur-containing alkoxysilanes is greater than or equal tothe quantity of polysulphide additives of the formula (I).
 7. Thesilica-containing rubber mixture according to claim 1, wherein therubber comprises at least one SBR rubber and at least one BR rubber. 8.The silica-containing rubber mixture according to claim 7, wherein therubber also comprises at least one NR rubber.
 9. The silica-containingrubber mixture according to claim 1, wherein the mixture comprises 1 to15 parts by weight of one or more sulphur-containing alkoxysilanes,based on 100 parts by weight of the rubber, and a ratio by weight ofsulphur-containing alkoxysilane to polysulphide additive of the formula(I) is 1.5:1 to 20:1.
 10. The silica-containing rubber mixture accordingto claim 1, wherein the mixture comprises 0.3 to 7 parts by weight ofthe polysulphide additives of the formula (I), based on 100 parts byweight of the rubber, and 0.3 to 7 parts by weight, based on 100 partsby weight of the rubber, of1,6-bis(N,N-dibenzylthiocarbamoyldithio)hexane (CAS No.: 151900-44-6).11. to the silica-containing rubber mixture according to claim 1,wherein the mixture comprises 0.5 to 5 parts by weight, based on 100parts by weight of the rubber, of the polysulphide additive of theformula (I) and 0.5 to 5 parts by weight, based on 100 parts by weightof the rubber, of 1,6-bis(N,N-dibenzylthiocarbamoyldithio)hexane (CASNo.: 151900-44-6).
 12. The silica-containing rubber mixture according toclaim 1, wherein the filler comprises one or more inorganic and/ororganic fillers, where a quantity of the fillers is 50 to 200 parts byweight, based on 100 parts by weight of the rubbers.
 13. Thesilica-containing rubber mixture according to claim 12, wherein thefiller is selected from the group consisting of oxidic fillers,silicatic fillers, carbon black fillers, and mixtures of these.
 14. Thesilica-containing rubber mixture according to claim 13, wherein thefiller is selected from the group consisting of precipitated silicasand/or silicates with specific surface area of 20 to 400 m²/g.
 15. Amethod for producing vulcanisates or rubber mouldings, the methodcomprising vulcanizing the silica-containing rubber mixture according toclaim
 1. 16. Vulcanisates or rubber mouldings comprisingsilica-containing rubber mixtures according to claim
 1. 17. A processfor the production of the silica-containing rubber mixture according toclaim 1, the process comprising mixing the rubber, thesulphur-containing alkoxysilane, the crosslinking agent, the filler, theat least one polysulphide additive, and the1,6-bis(N,N-dibenzylthiocarbamoyldithio)hexane (CAS No.: 151900-44-6).18. An additive mixture comprising: 10 to 90 parts by weight, based on100 parts by weight of additive mixture, of1,6-bis(N,N-dibenzylthiocarbamoyldithio)hexane (CAS No.: 151900-44-6),and 10 to 90 parts by weight of one or more polysulphide additives ofthe formula (I), based on 100 parts by weight of additive mixtureA-S—(S)_(x)—S—B  (I) where x is 0, 1, 2, 3 or 4, A is a moiety

and B is a moiety

where R¹to R⁴ are identical or different and are hydrogen,C₁-C₆-cycloalkyl, C₆-C₁₀-aryl or a group —CH₂—OR⁵, —CH₂—CH₂—OR⁵, —NHR⁵,—COR⁵, —COOR⁵, —CH₂COOR⁵, where R⁵=hydrogen, C₁-C₆-alkyl,C₅-C₆-cycloalkyl, C₆-C₁₀-aryl or C₁-C₆-acyl, R⁶ to R⁷ are identical ordifferent and are hydrogen, C₁-C₆-alkyl, C₅-C₆-cycloalkyl, C₅-C₁₀-arylor a group —CH₂—OR⁵, —CH₂—CH₂—OR⁵, —NHR⁵, —COR⁵, —COOR⁵, —CH₂COOR⁵,where R⁵=hydrogen, C₁-C₆-alkyl, C₅-C₆-cycloalkyl, C₆-C₁₀-aryl orC₁-C₆-acyl, and y and z independently of one another are 0, 1 or 2, or Aand B independently of one another are one of the radicals

in which R¹ and R² are identical or different and are hydrogen,C₁-C₆-alkyl, C₅-C₆-cycloalkyl, C₆-C₁₀-aryl or a group —CH₂—OR⁵,—CH₂—CH₂—OR⁵, —NHR⁵, —COR⁵, —COOR⁵, —CH₂COOR⁵, where R⁵=hydrogen,C₁-C₆-alkyl, C₅-C₆-cycloalkyl, C₅-C₁₀-aryl or C₁-C₆-acyl.